In situations when it is simply too dark to photograph, you’ll need to use the flash to provide all of the light that illuminates the subject. While it is possible to us flash as main light in your DSLR camera program mode, strongly recommend working in manual exposure so that you can control the tonality of the background.
Here is an example: Let’s say that you have a tame tricolored heron in your sights early on a cloudy-black morning. You’d like to make a head-and-shoulders portrait of the bird against dark green water, so you add your 1.4X teleconverter – and are dismayed to see that your shutter speed is only 1/25 sec at the wide-open aperture of f/5.6. This is the perfect situation to use flash as main light.
Set your DSLR camera to manual mode and select 1/60 sec as the shutter speed and f/5.6 as the aperture. This represents an underexposure of 1
Now set the flash exposure so that the subject is properly exposed by the flash. With film, setting the flash at zero compensation (0 EV) will generally yield good results. Theoretically, the same should be true when using flash with digital, but in low-light situations, such as our heron example, flash levels ranging from -0.3 to -1 EV will properly illuminate the subject. Teaching folks to check their digital exposures by looking at the histogram rather than at the LCD display, but when using flash, viewing the image on the LCD can quickly reveal whether or not you over flashed the subject.